1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
As a conventional image forming apparatus such as a copying machine, a laser beam printer, a facsimile machine, a word processor and a multi functional machine thereof, there is one which forms an image on a sheet using an electrophotographic system. In such an image forming apparatus, after an image is formed on one side (first side) of a sheet by an image forming portion, the sheet is again conveyed to the image forming portion, and an image is formed on the other side (second side) of the sheet.
To form an image on the second side of the sheet, the image forming apparatus which forms an image on both sides of a sheet includes a turn-over apparatus, a re-conveying path and a re-conveying roller to switch back a sheet having an image formed on its first side and to turn over the sheet. For example, an image forming apparatus such as a small printer includes, as the turn-over apparatus, a reversely-rotatable roller which can rotate normally and reversely, and when an image is formed on the second side of the sheet, the reversely-rotatable roller is rotated normally and reversely, and the sheet is conveyed to the re-conveying path while switching back the sheet (see Japanese Patent Application Laid-open No. 2000-109275). As the conventional image forming apparatus, to enhance the productivity when a sheet is turned over, there is an image forming apparatus in which the reversely-rotatable roller is driven by a motor capable of rotating normally and reversely, and a pulling velocity is changed by switch back according to a receiving velocity of the sheet (see Japanese Patent Application Laid-open No. 2006-56682).
In the case of an image forming apparatus such as a small printer, generally, a sheet is conveyed by a fixing device having a stable conveying force so that an image can be formed stably without depending on an image pattern and types of sheets. To suppress a curl of a sheet generated by heat applied to the sheet at the time of a fixing operation, the reversely-rotatable roller is made to convey the sheet at a faster velocity than the fixing device, the sheet is pulled and the curl is corrected (see Japanese Patent Application Laid-open No. 61-75369). In the case of such an image forming apparatus, a sheet conveying velocity is set such that the following relation is established: conveying roller velocity (before forming an image)≈(substantially equal) image forming velocity≦fixing velocity<reversely-rotatable roller velocity, so that an image can be formed stably and a curl can be corrected.
In the conventional image forming apparatus such as the small printer, when an image is formed on a first side, non-fixed toner adheres to a sheet in some cases. To form an image on a second side, if the sheet to which toner adheres is conveyed to the re-conveying path, the toner on the sheet adheres to the re-conveying roller provided on the re-conveying path, and an amount of toner adhering to the re-conveying roller is increased according to the number of sheets which pass.
As described above, the sheet conveying velocity of each sheet conveying portion has the relation of a conveying roller velocity (before forming an image)≈image forming velocity fixing≦velocity<reversely-rotatable roller velocity. A velocity relation between the reversely-rotatable roller and the conveying roller which are driven by the normally and reversely rotatable motor and which carry out switch back of a sheet is reversely-rotatable roller>>conveying roller. Here, if a velocity of the re-conveying roller which conveys a sheet which is switched back by the reversely-rotatable roller to the conveying roller is set to satisfy a relation of reversely-rotatable roller>re-conveying roller>conveying roller, a sheet forms a loop between the re-conveying roller and the conveying roller by a velocity difference.
If a sheet forms a loop, even after a rear end of the sheet passes through the re-conveying roller, the rear end of the sheet abuts against a surface of the re-conveying roller and slips for a while until the loop is eliminated. At that time, if toner of a certain level or more adheres to the re-conveying roller, toner adheres to the rear end of the sheet from the re-conveying roller due to this slip. As a result, a stain is generated on the rear end of the sheet during the re-feeding operation of the sheet, i.e., a tip end of the first side at a location which is contact with a roller outer periphery of the re-conveying roller. For example, a band-shaped stain having a roller width of 2 to 4 mm of the re-conveying roller is generated on the tip end of the first side.
If the velocity is set to satisfy a relation of re-conveying roller velocity<conveying roller velocity so that a sheet does not form a loop between the re-conveying roller and the conveying roller, a relation of reversely-rotatable roller velocity>>>re-conveying roller velocity is established, and an excessively large loop is generated between the reversely-rotatable roller and the re-conveying roller. If the excessively large loop is generated, the sheet is formed into an accordion shape at a location of the re-conveying path upstream of the re-conveying roller in the sheet conveying direction, a printing side of the sheet rubs against an inner surface of a conveying guide constituting the re-conveying path, and an image stain is generated. Since it is difficult to control a loop in the conveying guide, a sheet cannot be delivered to the re-conveying roller in an orderly fashion due to a curl generated at the fixing portion, and this can cause a paper jam.
The present invention has been accomplished in view of such circumstances, and the invention provides an image forming apparatus capable of forming images on both sides of a sheet without generating an image stain.